PartyKit multiplayer

Real-time multiplayer for studio games. The transport is PartyKit (a Cloudflare Durable Object behind a WebSocket), wrapped behind a thin typed client so the rest of the engine never touches the socket. Networking lives in two places:

• party/<room>.ts — the server: one Durable Object class per room (the "party").
• src/net/* — the client: NetClient (typed event interface over partysocket), RemoteAvatar (the visual + raycast proxy for another player), and playerAvatars.ts (shared avatar/slot-color data).

The networked game mode is a normal system in modes/ (see fps-arena): it owns the NetClient, maps server messages onto the world, and pushes scoreboard state to the HUD.